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Phy 121
Your 'cq_1_15.1' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
** CQ_1_15.1_labelMessages **
A rubber band begins exerting a tension force when its length is 8 cm. As it is stretched to a length of 10 cm its tension increases with length, more or less steadily, until at the 10 cm length the tension is 3 Newtons.
•Between the 8 cm and 10 cm length, what are the minimum and maximum tensions?
answer/question/discussion: ->->->->->->->->->->->-> :
I would say the minimum is 0 Newtons and the maximum is 3 Newtons.
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•Assuming that the tension in the rubber band is 100% conservative (which is not actually the case) what is its elastic potential energy at the 10 cm length?
answer/question/discussion: ->->->->->->->->->->->-> :
I think it would be 0 at the 10 cm length and then when it is released, the conservative energy will equal what it was when it originally started.
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You're on the right track thinking in terms of the conservative force.
You need to find the work done by the conservative force when the rubber band is stretched. The elastic potential energy will be equal and opposite to this work.
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•If all this potential energy is transferred to the kinetic energy of an initially stationary 20 gram domino, what will be the velocity of the domino?
answer/question/discussion: ->->->->->->->->->->->-> :
I am assuming this is a different scenario in which the Fnet is still 3 Newtons. I am going to convert g to kg. 20 g * (1 kg / 1000 g) = 0.02 kg
Therefore we now know the Fnet and m and can find a
Fnet = m * a
a = Fnet / m = 3 Newtons / 0.02 kg = 150 m/s^2
At this point we know v0, a, and nothing else to my knowledge, so I am stumped on what to do next.
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You also know `ds, which is the 2 cm displacement.
However you would need to base your result on the average force, not the maximum force.
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The key to solving this problem, though, is to use conservation of energy.
Having calculated the PE of the system, you would assume that it is converted to KE. Knowing that the initial KE is zero (the domino is initially at rest) you would therefore know the mass and the KE, from which you can find the velocity.
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•If instead the rubber band is used to 'shoot' the domino straight upward, then how high will it rise?
answer/question/discussion: ->->->->->->->->->->->-> :
I'm not sure what to do here either. I know we are looking for `ds but from just knowing Fnet, a, and m, I am not sure where to go next to find the answer.
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You would use energy conservation to answer this question.
You know the elastic PE of the system.
When the domino is shot straight up, this elastic PE is converted to gravitational PE.
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You tend to revert to analysis of uniformly accelerated motion rather than using energy conservation. This is natural, given that you are adept at this sort of analysis. However energy conservation allows us to bypass that analysis, which (once you're used to thinking in these terms) actually simplifies the solution. It also applies to situations in which acceleration is neither uniform nor linear in nature, where analysis of the detailed motion is not possible.
This is worth 15-20 more minutes of your time. See what you can do within this time frame, and submit what you have. At that point I'll link you to the full discussion of this situation.
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